Method of operating a lithographic apparatus, device manufacturing method and associated data processing apparatus and computer program product

Abstract

A reticle is loaded into a lithographic apparatus. The apparatus performs measurements on the reticle, so as to calculate alignment parameters for transferring the pattern accurately to substrates. Tests are performed to detect possible contamination of the reticle or its support. Either operation proceeds with a warning, or the patterning of substrates is stopped. The test uses may use parameters of the alignment model itself, or different parameters. The integrity parameters may be compared against reference values reflecting historic measurements, so that sudden changes in a parameter are indicative of contamination. Integrity parameters may be calculated from residuals of the alignment model. In an example, height residuals are used to calculate parameters of residual wedge (Rx′) and residual roll (Ryy′). From these, integrity parameters expressed as height deviations are calculated and compared against thresholds.

What is claimed is: 
     
       1. A method comprising:
 measuring features of a patterning device supported in a lithographic apparatus, the results of the measuring being used in normal operation in calculating alignment parameters for use in controlling transfer of a pattern to one or more substrates; and 
 performing, on the basis of the measuring, an integrity test to identify a contaminated condition of the patterning device or its support, 
 wherein the integrity test comprises determining if one or more calculated integrity parameters are above a threshold, the threshold being determined based on reference data related to the patterning device, 
 wherein the integrity test uses a subset of the alignment parameters that are non-correctable parameters, and 
 wherein each of the non-correctable parameters represents a modeled component transformation of the pattern. 
 
     
     
       2. The method of  claim 1 , wherein:
 the performing comprises identifying different severity levels of contaminated conditions; 
 for a first severity level, operation of the lithographic apparatus proceeds while a warning is displayed or recorded; and 
 for a second severity level, operation of the lithographic apparatus is interrupted or not started. 
 
     
     
       3. The method of  claim 1 , wherein:
 the alignment parameters include coefficients of a polynomial alignment model mapping local deviations in the plane of the pattern, and 
 the integrity test is based at least partly on values of one or more coefficients selected from the coefficients of the polynomial alignment model. 
 
     
     
       4. The method of  claim 1 , wherein the alignment parameters include wedge and roll parameters, and the integrity test uses one or both of the wedge and roll parameters. 
     
     
       5. The method of  claim 1 , wherein the integrity test is based at least partly on residual values of the alignment measurements, the residual values being the difference between measured values and modeled values of the alignment parameters. 
     
     
       6. The method of  claim 5 , wherein the residual values represent the difference between height values measured at a number of points on the patterning device, and height values represented by the alignment model. 
     
     
       7. The method of  claim 1 , further comprising initiating further inspection or cleaning of the patterning device in response to detection of a contaminated condition. 
     
     
       8. The method of  claim 1 , further comprising:
 making measurements of an applied pattern on one or more substrates, 
 defining a modified integrity test based on the measurements of the applied pattern, and 
 using the modified integrity test in future performances of the method. 
 
     
     
       9. A non-transitory computer-readable storage medium having instructions stored thereon, execution of which by a processor cause the processor to perform operations, the operations comprising:
 measuring features of a patterning device supported in a lithographic apparatus, the results of the measuring being used in normal operation in calculating alignment parameters for use in controlling transfer of a pattern to one or more substrates; and 
 performing, on the basis of the measuring, one or more integrity tests to identify a contaminated condition of the patterning device or its support, 
 wherein the one or more integrity tests comprises determining if one or more calculated integrity parameters are above a threshold, the threshold being determined based on reference data related to the patterning device, 
 wherein the integrity test uses a subset of the alignment parameters that are non-correctable parameters, and 
 wherein each of the non-correctable parameters represents a modeled component transformation of the pattern.